mm/page_poison: remove unused page_ext.h from page_poison

Patch series "minor cleanups to page_ext header".

No page_ext function or structure is used in page_poison.  Just remove
page_ext header from page_poison.

Link: https://lkml.kernel.org/r/20230717113227.1897173-1-shikemeng@huaweicloud.com
Link: https://lkml.kernel.org/r/20230717113227.1897173-2-shikemeng@huaweicloud.comSigned-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page_poison.c

@@ -4,7 +4,6 @@
 #include <linux/mm.h>
 #include <linux/mmdebug.h>
 #include <linux/highmem.h>
-#include <linux/page_ext.h>
 #include <linux/poison.h>
 #include <linux/ratelimit.h>
 #include <linux/kasan.h>